1. Field of the Invention
The present invention relates to a driving circuit for an analog luminescent display tube which analogously displays numerical information digitally given.
2. Description of the Prior Art
In order to represent or display information concerning quantity, position and the like, analog representation or display is adopted in many cases because analog representation has advantages in that it can easily grasp the ratio of a quantity to the whole or the position in question.
For the purpose of performing the above-mentioned analog representation or display, various types of devices have been heretofore developed and put into practical use. In recent years, a luminescent display tube for analog display (hereinafter referred to as luminescent display tube) has been put into practical use, which uses luminescent material capable of luminescing when bombarded with electrons.
The luminescent display tube utilizing the luminescence of luminescent material has a substrate or base plate, a plurality of segment anodes arranged on the base plate, for instance, rectilinearly and each coated with a luminescent material layer, a mesh-like control grid provided near and above the segment anodes, and a filament cathode or cathodes provided above the control grid (hereinafter referred to as grid) and adapted to emit thermions when heated. In this luminescent display tube, anode voltage is given to the segment anodes corresponding in number to the input information to be displayed, and thereby the segment anodes are made to luminesce and accordingly the input information is displayed in the form of a luminescent bar-shaped pattern. Thus, analog representation can be performed.
The luminescent display tube of this type provides high-quality color of luminescence and therefore can be used in a dark environment such as at night; in addition, it has no moving parts, and therefore is free of malfunctions caused by mechanical vibrations and can be operated at high response speed.
In order to drive the above-mentioned luminescent display tube for use in analog display, the following driving circuits are usually adopted.
In an example of the driving circuit, a plurality of segment anodes each coated with a luminescent material layer are connected independently of each other to external terminals through which anode voltage is given to the segment anodes corresponding in number to the input information to perform static analog display.
According to the above driving circuit, it is possible to make the segment anodes luminesce by a relatively low anode voltage. However, it has disadvantages in that the increase of the number of segment anodes for the purpose of increasing the amount of display or improving the reading accuracy will cause the increase of the number of external terminals and that of the number of driving circuits which give driving voltage to the segment anodes.
In another example of the driving circuit, a number of segment anodes arranged rectilinearly and each covered with a luminescent material layer are grouped into a plurality of groups each having the same number of segment anodes; the corresponding segment anodes of the respective groups are connected in common to the external terminals, respectively; and a plurality of grids are provided for the respective groups independently of one another, being connected to the external terminals separately. The above grids are scanned in regular succession and in a time-sharing manner, and the segment anodes are given driving voltage in synchronization with the scanning signal for the grids according to the input information so that display may be performed.
According to the above driving circuit, the number of the external terminals becomes the sum of the number of the segment anodes of each group and that of the grids provided for the respective groups. Therefore, even when the number of segment anodes of each group or that of the groups is increased, the increase of the number of external terminals can be minimized; accordingly, fine display can be performed with a small number of external terminals.
In the latter example of the driving circuit, however, the increase of the number of groups results in the decrease of the duty factor of each segment anode, i.e., the ratio of the duration of luminescence of each segment to the scanning period of the grid. Therefore, the driving voltage given to the segment anode must be kept high in order to obtain desired luminance.